The effects of non-parallel plates in a differential capacitive microaccelerometer

The effects of non-parallel plates on the capacitance, sensitivity, electrostatic force and electrostatic spring constant of a microaccelerometer are investigated. In a deep reactive ion etching similar to the single crystal reactive ion etching and metallization (SCREAM) micromachining process, the width of the fingers increases linearly with depth and the cross section takes on a trapezoidal profile. This non-ideal feature is greater in high aspect ratio structures and significantly affects the capacitance, sensitivity, electrostatic force and electrostatic spring constant of the accelerometer operated in either closed or open loop. Finite-element analyses (FEA) of the capacitance and electrostatic force of the parallel-plate and non-parallel-plate models were completed and the results were compared with those derived from the theoretical approach. It is verified that the capacitance, sensitivity, electrostatic force and electrostatic spring constant of the non-parallel-plate model are larger than the parallel-plate model. Hence this non-ideal feature should not be neglected and the assumption of parallel-plate model would give an underestimate of these physical parameters.